The present disclosure relates generally to the logging of subsurface formations surrounding a wellbore using one or more well logging instruments, and particularly to making disparate measurements with the one or more instruments while drilling and using those disparate measurements to infer one or more formation properties. More specifically, the present disclosure relates to techniques for determining formation properties through solving combined well logs using graphical methods, such as Sigma-resistivity logs.
Well logging instruments have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the wellbore such as the mineral composition and fractional volume of pore space (“porosity”) as well as properties of fluids in porous formations. Well logging instruments known in the art include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other types of tools are also used.
Early logging instruments were run into a wellbore on an armored electrical cable (“wireline”) after the wellbore had been drilled. Modern versions of such wireline instruments are still used extensively. However, the need for information while drilling the wellbore gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide measurements of drilling parameters such as weight on the bit, torque, temperature, pressure, wellbore geodetic or geomagnetic direction, and wellbore inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions (e.g., T1 and T2). MWD and LWD tools may have sensing components in common with wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to operate in the harsh environment of drilling, frequently for extended periods of time.
Some logging instruments may be combined to different types of formation measurements in a single “pass” or “run” of well logging instruments through the wellbore. By way of example, a combined instrument may be capable of measuring formation total porosity (Φ), thermal neutron capture cross section (Σ) and electrical resistivity (R) simultaneously. For example, a combination instrument may be one sold under the trademark ECOSCOPE, which is a trademark of Schlumberger Technology Corporation, Sugar Land, Tex. The inclusion of a plurality of different sensors on a combination logging instrument allows the measurements to be made in substantially in the same formation zone of investigation and substantially at the same time.
Sometimes, specific multiple measurement instrument are combined to enable determining certain formation parameters from the particular combination. For example, Sigma (S) and resistivity (R) measurements may be combined to compute Sw, and Rw, curves (where the subscript “w” denotes formation water) using numerical techniques to obtain Sw and Rw that are consistent with the measurements. Examples of such numerical techniques are described U.S. Pat. No. 8,441,269 issued to Minh et al. However, it would be useful to provide a graphical method for determining formation parameters from multiple types of well log measurements, with the graphical method being available for use instead of or in addition to existing numerical methods.